| Over the past few years the semiconductor industry has made a very strong push towards the use of optical enhancements to enable printing of features beyond the resolution limits of diffraction-limited lithographic projection systems. It is generally agreed that advanced mask technologies such as optical proximity correct (OPC) and phase-shifting masks (PSM's) are needed. Both of these techniques require the development of fast automated design algorithms, for transition from process research and development to production manufacturing use.; In this thesis we describe a computationally efficient mask design algorithm based on alternating projection methods. We first develop the optimal coherent decomposition (OCD) method to approximate complicated partially coherent systems with simple coherent systems. Then, we cast the mask design problem as a classical phase-retrieval problem in optics, and construct a close relative of the well-known Gerchberg-Saxton algorithm to design masks for arbitrary IC patterns. To control mask complexity, we propose a double-exposure strategy that produces a desired pattern by exposing two simple two-phase masks sequentially, and the half-toning methods that enable closer approximations to continuous modulation of amplitude by the mask.; The algorithm described in this thesis has been experimentally verified using a variety of test patterns ranging from simple u-shaped patterns to more complex gate-array and SRAM patterns. |
